Kin selection theory was developed by W. D. Hamilton in 1964. It pointed out that there is a hidden genetic benefit in being kind to one's offspring and close genetic relatives. A gene for kindness to relatives can prosper because it tends to help other copies of the same gene to prosper, copies that happen to be in bodies other than one's own. They are there because they were passed down from the same recent common ancestor to both oneself and one's relatives. Thus, generosity to blood relatives is actually genetic selfishness.

This can be hard to understand, so let's view evolution from a selfish gene's point of view. Genes are "selfish" in the sense that they evolve to generate as many copies of themselves as possible. They act as if they are trying to spread throughout a population. Often, they do so by constructing bodies and brains that act as self-interested individuals. But that is by no means the only way for a selfish gene to spread. All genes would profit from being able to recognize and help copies of themselves in other bodies, and would be able to spread themselves better if they could. But most of them cannot, because they help only to grow lungs, or livers, or some other blind organ incapable of generous behavior. Only a few kinds of genes have the power to be selectively generous to blood relatives. Some such genes are involved in growing a perceptual ability to distinguish kin from non-kin, or a behavioral ability to be kinder to kin. Others may grow nutrient-delivery systems such as wombs and breasts that nourish another individual that happens to be growing inside one's body, or clinging to oneself, which suggests they are probably one's own offspring. These are the main ways in which kindness-to-kin genes evolve.

Kin selection is really just a theory about how a particular kind of genetic mutation can spread. The only mutation that can assist other copies of itself in other bodies is a mutation for recognizing and favoring close genetic relatives. By favoring close relatives, who share a recent common ancestor, they are most likely to be favoring individuals who happen to carry a copy of this kin-favoring mutation. The whole edifice of family life, kinship, and parenting is based on a few very peculiar mutations that are directly responsible for kin recognition and nepotism. These kin recognition genes single-mindedly evolve better ways to recognize and assist one another, while all the other genes go about their business oblivious to kinship.

Kin selection predicts that we should be kinder to relatives who are more closely related to us: our altruism to kin should be in proportion to the likelihood of sharing the altruism gene with them. That likelihood is determined by how recently we shared a common ancestor, and how many common ancestors we shared. The likelihood of sharing the same kindness-to-kin gene is one half for siblings, parents, and offspring, but only a quarter for half-siblings, grandparents, grandchildren, nieces, nephews, aunts, and uncles. This is why we are usually kinder and closer to sisters than to nieces, even in societies where extended families live together.

A confusion commonly arises at this point. Kin selection theory is often misunderstood as saying we should be kind to other organisms in proportion to the true percentage of genes we share with them. Don't all humans share about 99 percent of our DNA? That sounds close to identical twins, who share 100 percent of their DNA. If we share so many of our genes with other humans, why should we discriminate between close relatives and distant relatives? And don't we share about half of our genes with other mammals, birds and even fish? We should treat all herring as brothers and all sloths as sisters. We should apply the golden rule to all primates, be true friends to all mammals, allies to termites and tapeworms, and just slightly grudging compatriots of baobab trees, stinging nettles, and Antarctic lichens. Universal peace, cooperation, and symbiosis should reign on our blessed planet, according to this genetic-similarity interpretation of kinship.

It would be a weird and wonderful world if there were an evolutionary process that could favor altruism in proportion to true genetic similarity. Racism, ethnocentrism, xenophobia, sexism, human competition, crime, warfare, deforestation, pollution, and cruelty to animals would all vanish. We would all behave like Jainists—members of the Indian religion who dare not eat or move for fear of injuring another living being. But there is no such evolutionary process. Kinship theory offers only a forgetful, myopic, fumbling imitation, in which we have evolved the delusion that only our extremely close relatives have anything genetic in common with us.

How much of human morality derives from kin selection? Opinions vary. W. D. Hamilton, E. O. Wilson, and many others have suggested that adaptations for kindness to kin may have been important building blocks for kindness toward non-kin. Kin selection does not require a brain, but having a brain helps. It was a major step for brains to evolve the abilities to recognize individual relatives, determine how much care they should receive based on cues of genetic similarity, and produce care behaviors that actually benefit them. It looks as though it should be fairly easy to modify such adaptations to recognize individual non-relatives, determine how much care they should receive based on other kinds of cues, and produce effective care behaviors.

But would evolution favor such modifications? The genes underlying kindness to kin could have evolved only if they discriminated against non-kin. Their success ever since they originated as rare little mutations has depended on them being selectively altruistic, not generally altruistic. Although psychologically it looks like a small step to extend kin-based altruism to non-kin, it is a huge evolutionary leap that violates the basic rationale of kin selection. Being able to imagine "all men are brothers" is a long way from acting as though they are. Kin recognition is widespread among mammals. If human morality evolved as a free-and-easy side-effect of kin-based altruism, we might expect most mammals to show human-like morality They don't. Apparently, evolution ruthlessly eliminates all kinship genes that lose their discriminative abilities and start treating strangers as relatives. Kinship is powerful at explaining our kindness toward blood relatives, but it is hard to extend it beyond that.

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